Oak moths mating

California Oak Moth

The California oak moth (Phryganidia californica) is a native insect of coastal California. When heavy outbreaks occur, they voraciously feed upon the leaves of our native Coast Live Oak and also some deciduous trees. The tan brown adult moths are commonly seen in the late afternoons as they hover around infested trees. They have only one goal which is to mate and lay eggs for the next generation of caterpillars. The moths themselves do not feed on the leaves. Extreme oakworm outbreaks occur approximately every 8-10 years. In the most extreme years, such as the one we experienced in 2007-08, the infestation may lead to severe oak defoliation. During these outbreaks caterpillars are often seen suspended from silk strands, dropping to the ground, and congregating on fence posts, mail boxes, and other available platforms.California Oak Moth Caterpillars on Fencepost

Healthy oaks usually recover from these defoliation events. Age, condition of trees, as well as human caused impacts must be considered. Oaks that are simultaneously subjected to stresses caused by landscape or home construction, drought, soil compaction, or pathogen infections (i.e. Sudden Oak Death Syndrome or oak root fungus) may be more seriously affected by oakworm defoliation. California oakworm infestations continued in many of our local areas.Leaf-Chewing Oak WormsLeaf Scraping

Our experience indicates that each outbreak usually lasts for two years.

Tree Solutions does not agree with blanket statements that say this is a native pest on native trees and no action is needed. We acknowledge that there is a symbiotic relationship and in natural undisturbed settings, control is not usually warranted. However, in urban environments, we believe that each tree must be individually considered. Growing environment conditions vary and tree health is often compromised in the urban environment. Many trees are stressed, and stressed trees require action.

Defoliated Tree LimbsEarly signs are difficult to detect. Tree Solutions recommends monitoring by a certified arborist who is experienced in insect and disease recognition. Early detection and timely treatment is advantageous in controlling the caterpillar outbreak. The pupae are white or yellow with black markings. The mature larvae are 3/4 to 1 inch long and are olive green with black and yellow longitudinal lines along the back and sides; they have a reddish brown head.

Tree Solutions employs an Integrated Pest Management (IPM) approach for oak worm control. It begins with site monitoring followed by action if necessary. When unacceptable pest populations build, control strategies are employed. When chemical control is required, we use an array of reduced risk pesticides. We have been using the biological control, Bt. (Bacillus thuringiensis) and Pyrethrum which is derived from Chrysanthemum flowers for canopy sprays. Bt. requires two applications and proper timing is essential Abamectin, which is a natural fermentation product of a soil bacterium, is formulated by pest control material manufacturers and used for trunk injections. This application method eliminates environmental exposure to pesticides. The use of this application method is also timing critical.

Although the low-toxicity pesticides we use have minimal impact upon the environment, and exposure will rarely cause health problems, when properly applied they offer sufficient control. They are pest specific, have short environmental residual times, do not bind with soil or move downward into aquifers. They have minimal impact on the beneficial insect populations associated with oak trees. There are chemicals being used by local companies that are highly toxic to both humans and wildlife. These full spectrum and persistent insecticides may continue to kill many organisms which are important to tree health for months after an application. Before hiring a pest control company, find out what material is used and research the product.

A Proactive Approach

Prior to the warmer weather of late winter and spring when caterpillars will begin to become most active, an injection treatment using a caterpillar specific insecticide may prevent an uncontrollable outbreak. The material is dispersed throughout the tree canopy into its leaves. When a caterpillar feeds on a leaf it ingests the insecticide and control is achieved. In many cases, this treatment will eliminate the need for future canopy sprays. Up to 90% control may be achieved. We use this control on those trees which were infested and defoliated for the first time during the previous year. We do this because in our experience, infestations usually last for two years.

Oak Moth TrapsIf there is caterpillar activity at the time of injection, we recommend a concurrent canopy spray. Trees that respond the best to this treatment are healthy and vigorous. Trees with trunk and branch defects or trees that are growing in a tree-hostile environment, will achieve a degree of control, but usually not complete control. Subscribing to the monitoring program will help us keep your trees green and healthy.

Oak Moth Frass on the GroundWhen oak moth caterpillars begin feeding they drop frass. Their frass is small and golden brown in color. When frass begins to build, it is time to spray. Bt. spray programs require two timed applications spaced approximately ten days apart. We suggest that beginning mid July  you place a white paper plate beneath the canopy of your oak tree and monitor it frequently for frass droppings. When frass begins to build, call us for treatment.

Defoliated TreeIf you are on the monitoring program, we will make timely visits to monitor for activity; you will not need to contact us. We will inform you when treatment is necessary. There are usually two and occasionally three caterpillar generations during the year so monitoring is recommended.

During drought years, deep soaking of the root zone with water will help reduce tree stress. Deep watering should occur approximately once a month and it needs to reach the fine feeder roots which are located approximately 6-12 inches below the soil surface. Tree Solutions will help you determine an optimum watering schedule and method of application. Usually soaker hose or inline emitter hose is preferred to oscillating sprinklers. This is because water should not spray onto the trunks of trees to help prevent root collar diseases.

Sign up for our monitoring program. We we will make several timely visits to your home to observe caterpillar populations and begin treatment when it becomes necessary. We will also make cursory exams and notify you of other tree problems.

Contact us at:

  • 831-425-1833 in the Santa Cruz/Monterey area, or
  • 650-269-0540 in the Silicon Valley/Bay Area.

Certified Arborist WE-6717A

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